OP, your post really really resonated. I would second counselling. You need to be in control and part of that is understanding what buttons your mother pushes and why you react as you do.
I was "lucky" in that a few months before the mother problems really kicked off, I was being made very unhappy in a voluntary context by someone who was behaving very strangely and, essentially, picking on me. Friends were happy to agree she was rather odd, but the issue was why I reacted as I did. And the way I reacted seemed to single me out for more such treatment. The lightbulb moment was realising that I reacted to the manipulation and criticism in the same way as I would react to my mother. Taking the criticism passively, getting very upset, trying to please etc.
Then a bit of Googling around Grandiosity, Narcissism, and also Eric Berne's Transactional Analysis parent adult child model.
Its not going to get easier. The relationship model used at the moment where your mother is the parent and you are the child is wearing you down. One thing about elderly parents is that we start caring for them, and though we will aim for as much adult to adult interaction, and indeed still want to be the child, there will be many times,. if things are to be sustainable, when you will need to take on the parent role and make the decisions.
So step back, and focus on you. You need to work out what is going on and how you can constructively assert yourself. You need to decide what boundaries you need and how to defend them. You also need to learn that you are not responsible for things you cannot control.
I ended up going with the flow when my dad was terminally ill. My mother was getting exhausted, and became willing to at least hear me out, when I suggested I attend a MacMillan nurse visit, though my dad then died suddenly in his sleep before things got any worse. I wanted to do this for my dad, and he took comfort from me being there, even though it was exhausting.
Thereafter:
- I decided that I, as an individual, wanted to do "the right thing" and that this was an individual decision regardless of the behviour of others, past or present.
- I decided that my boundaries were at the point where either my husband or my children would start feeling resentful (time or money or emotions).
- I started not accepting some elements of my mother's behaviour. Quietly and firmly. For example I stopped her when she came out with a load of nasty criticism in front of the children. She was genuinely quite shocked. Now I generally ignore it, and more normally takes the form of her telling anyone who will listen how dreadful I am, rather than being aimed at me directly. (Though it is still upsetting!) I also ignore her criticism of others (and "common" is only the starting point..) though now note that a sign that her dementia is getting worse is that she is losing the ability to differentiate between family, who she can be openly rude to, and others, who she is generally polite to, at least to their faces.
- I finally worked out that I could not do anything about things I could not influence. I did a little behind her back, like speak to the Inland Revenue to explain why they had not received a tax return in four years,. and then fax over a rough estimate of her income so they could call her in and "do" a tax return with her. But I gave up ringing round when she did not answer the phone. She would not tell me where a key was kept, so not much I could do. I visited her reasonably regularly, but did not eat in her flat as it was too filthy, or rather I picked up M&S food on the way for us both. Only when she had a fall, and she agreed to a POA, was I able or willing to sort things out.
FWIW, I'm glad I have stuck with it. There was a temptation to walk away. And reestablishing the relationship on a more sustainable basis was pretty brutal. Our relationship is better now than it has ever been, and as more people are involved in my mother's care, I have had some useful external recognition that the behaviour that upsets me so much, is unreasonable. Plus I am less likely to see myself through my mother's critical eyes.
In the short term, decide what you are willing to do and stick to it. When my dad was ill I was driving 100+ miles each way and only staying for a short time, as the atmosphere was so awful. She was not letting me help anyway. If need be, make something up. Say you fell and hurt your back and the doctor said you should not drive longer distances. The one that always worked for my mother was that DH was unhappy. She thought it normal that husbands would demand priority over my time.